Fuel cells are devices that directly convert chemical energy of reactants, i.e., fuel and oxidant, into direct current (DC) electricity. For an increasing number of applications, fuel cells are more efficient than conventional power generation, such as combustion of fossil fuel and more efficient than portable power storage, such as lithium-ion batteries.
In general, fuel cell technologies include a variety of different fuel cells, such as alkali fuel cells, polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells and enzyme fuel cells. Today's more important fuel cells can be divided into three general categories, namely (i) fuel cells utilizing compressed hydrogen (H2) as fuel, (ii) proton exchange membrane (PEM) fuel cells that use methanol (CH3OH), sodium borohydride (NaBH4), hydrocarbons (such as butane) or other fuels reformed into hydrogen fuel, and (iii) PEM fuel cells that can consume non-hydrogen fuel directly or direct oxidation fuel cells. The most common direct oxidation fuel cells are direct methanol fuel cells or DMFC. Other direct oxidation fuel cells include direct ethanol fuel cells and direct tetramethyl orthocarbonate fuel cells.
Compressed hydrogen is generally kept under high pressure, and is therefore difficult to handle. Furthermore, large storage tanks are typically required, and cannot be made sufficiently small for consumer electronic devices. Conventional reformat fuel cells require reformers and other vaporization and auxiliary systems to convert fuels to hydrogen to react with oxidant in the fuel cell. Recent advances make reformer or reformat fuel cells promising for consumer electronic devices. DMFC, where methanol is reacted directly with oxidant in the fuel cell, is the simplest and potentially smallest fuel cell, and also has promising power application for consumer electronic devices.
DMFC for relatively larger applications typically comprises a fan or compressor to supply an oxidant, typically air or oxygen, to the cathode electrode, a pump to supply a water/methanol mixture to the anode electrode, and a membrane electrode assembly (MEA). The MEA typically includes a cathode, a PEM and an anode. During operation, the water/methanol liquid fuel mixture is supplied directly to the anode, and the oxidant is supplied to the cathode. The chemical-electrical reaction at each electrode and the overall reaction for a direct methanol fuel cell are described as follows:
Half-reaction at the anode:CH3OH+H2O→CO2+6H++6e−
Half-reaction at the cathode:O2+4H++4e−→2H2O
The overall fuel cell reaction:CH3OH+1.5O2→CO2+2H2O
Due to the migration of the hydrogen ions (H+) through the PEM from the anode through the cathode and due to the inability of the free electrons (e−) to pass through the PEM, the electrons must flow through an external circuit, which produces an electrical current through the external circuit. The external circuit may be any useful consumer electronic devices, such as mobile or cell phones, calculators, personal digital assistants, laptop computers and power tools, among others. DMFC is discussed in U.S. Pat. Nos. 5,992,008 and 5,945,231, which are incorporated by reference in their entireties. Generally, the PEM is made from a polymer, such as Nafion® available from DuPont, which is a perfluorinated sulfuric acid polymer having a thickness in the range of about 0.05 mm to about 0.50 mm, or other suitable membranes. The anode is typically made from a Teflonized carbon paper support with a thin layer of catalyst, such as platinum-ruthenium, deposited thereon. The cathode is typically a gas diffusion electrode in which platinum particles are bonded to one side of the membrane.
As discussed above, for other fuel cells fuel is reformed into hydrogen and the hydrogen reacts with oxidants in the fuel cell to produce electricity. Such reformat fuel includes many types of fuel, including methanol and sodium borohydride. The cell reaction for a sodium borohydride reformer fuel cell is as follows:NaBH4+2H2O →(heat or catalyst)→4(H2)+(NaBO2)H2→2H++2e− (at the anode)2(2H++2e−)+O2→2H2O (at the cathode)Suitable catalysts include platinum and ruthenium, among other metals. The hydrogen fuel produced from reforming sodium borohydride is reacted in the fuel cell with an oxidant, such as O2, to create electricity (or a flow of electrons) and water byproduct. Sodium borate (NaBO2) byproduct is also produced by the reforming process. Sodium borohydride fuel cell is discussed in U.S. Pat. No. 4,261,956, which is incorporated herein by reference.
Valves are needed for transporting fuel between fuel cartridges, fuel cells and/or fuel refilling devices. The known art discloses various valves and flow control devices such as those described in U.S. Pat. Nos. 6,506,513 and 5,723,229 and in U.S. published application nos. 2003/0082427 and 2002/0197522. A need, however, exists for improved valves that allow venting of gas, maintaining seals, improving the flow of fuel through the valve, among other things.